Add get initial regs to ThreadUnwinder.

For debuggerd that wants to dump the initial registers of the thread
being dumped, add a way to copy those registers.

Add new unit tests to verify this register saving behavior.

Bug: 208933016

Test: Ran unit tests on host and device.
Change-Id: If84fd535654e3fb2a4612622aa595ca3f02c4a58
3 files changed
tree: 7b74d3c42065befea88a997e23e37b941d974ff3
  1. libbacktrace/
  2. libunwindstack/
  3. OWNERS
  4. PREUPLOAD.cfg